1. Field of the Invention
The present invention relates generally to a steering system for use in an automotive vehicle and, more particularly, to a steering system equipped with an air bag module for protecting a driver in the event of a vehicle collision and a tilt mechanism for tilting a steering shaft on which is mounted a steering wheel.
2. Description of the Prior Art
Conventionally, an automobile steering system equipped with a tilt mechanism for tilting a steering shaft so that the posture of a steering wheel may be properly adjusted in accordance with a driver's physique is generally known. Japanese Patent Laid-open Application (unexamined) No. 60-157962 discloses an electrically operated tilt mechanism provided with a steering shaft consisting of separated upper and lower main shafts. The lower main shaft is securely supported at a fixed angle with respect to the vehicle body, whereas the upper main shaft, on the rear end of which is mounted a steering wheel, can be tilted relative to the lower main shaft. The posture of the steering wheel with respect to a driver can be properly adjusted by electrically tilting the upper main shaft.
Generally speaking, when an air bag unit is mounted in an automobile steering system, at least an air bag or, generally, principal constituent elements such as, for example, an inflator (gas generator), an igniter (ignition unit) as well as the air bag are assembled into a casing as an air bag module, which is mounted in a steering wheel fixedly mounted on the rear end of a steering shaft. In this case, since the air bag module is generally rigidly secured to a steering spoke or a steering hub, the air bag module is integral with the steering wheel.
In the case of a vehicle collision, the air bag is pressurized to open the casing covering the air bag module and is inflated towards the driver, thereby protecting the driver from colliding against a front glass or the steering wheel.
In this case, if the air bag is inflated in the condition where the steering wheel is not tilted with respect to the steering shaft, i.e., along the center line of the steering shaft, the inflation of the air bag is most reliably directed to the driver's breast. In particular, even at the beginning of the inflation where the air bag cannot entirely cover the steering wheel, the air bag can reliably protect the driver's breast and will provide its shock absorbing ability to the full against the collision of the driver.
However, when the air bag module is mounted in the steering system equipped with the tilt mechanism and is integral with the steering wheel, as in the conventional case, the tilting operation of the steering wheel changes the direction of inflation of the air bag. In the case of a vehicle collision under such conditions, the air bag is inflated in a direction deviated from the center line of the steering shaft.
In view of this arrangement, a satisfactory shock absorbing effect can be maintained by changing the characteristic and specification of the air bag unit itself, for example by enlarging the capacity of the air bag. However, if the direction of inflation of the air bag can be kept unchanged even in the condition where the steering wheel is tilted, the air bag can reliably effectively restrict and protect the driver's breast even at the beginning of inflation of the air bag and the shock absorbing effect thereof can be further enhanced.
Furthermore, when the air bag module is mounted in the steering wheel of the tiltable steering system, the steering wheel is inevitably increased in weight, thereby lowering the operability of the tilt mechanism and causing the tilt mechanism to become large-sized.